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Electro-Oculography I Laboratory
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Electro-Oculography I Laboratory
Introduction
Sight is probably the most important of the five senses to many
human beings. Our visual system continuously provides feedback on
objects and interactions with our environment in almost everything
that we do. The eyes help us to focus on objects that are right in front
of us such as a book we are reading, they allow us to track a plane
shooting across the sky at an air show, and they allow us to focus on an
object that is far off in the distance. Several muscles are responsible for moving
the eyes around to complete these different types of tasks. Each of these distinct
tasks performed by the eyes requires specific types of controlled movements.
These movements each have their own name and will be explained in this
laboratory. Researchers have performed experiments to understand how the eye
functions, and the electro-oculogram (EOG) is one of the observed phenomena that can give us
insight.
Several theories exist on the exact mechanism responsible for generating the EOG potential.
However, regardless of its exact mechanism, the EOG is an important biopotential that can
provide us with information about the visual system. One important example where the EOG is
used is during overnight sleep studies (you will actually be completing an overnight sleep study
in a future lab). Sleep studies often include the EOG as one of many biopotentials that are
recorded. During certain portions of a sleep cycle, the eye movements become very erratic and
relatively large in magnitude. Monitoring and recording the EOG signal can help to detect these
stages of sleep. In addition, completing eye tracking exercises and monitoring the resulting EOG
can allow diagnosis of certain eye diseases. For example, an ophthalmologist can use the EOG
to diagnose retinal disorders that can lead to blurred vision.
In this lab, students will monitor and record the EOG signal from both the left and right eyes of a
subject. Various eye movement experiments, such as target tracking exercises, will be
performed. The data will be processed and analyzed to identify characteristics of and problems
with interpreting the EOG signal.
Equipment required:
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CleveLabs Kit
CleveLabs Course Software
Five snap electrodes and snap leads
Microsoft Excel
Measuring Tape
Microsoft Excel®, MATLAB®,
or LabVIEW™
Figure 1: The human eye is integral in supplying feedback to
the nervous system about the environment.
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Superior Rectus
Retina
Cornea
Fovea
Pupil
Optic Nerve
Iris
Lens
Inferior Rectus
Figure 2: Several anatomical structures of the eye are responsible for focusing on objects in the visual field.
Background
The human eyes contain several anatomical structures that function much like a camera and
allow us to focus on and track objects in the visual field (Fig 2). The cornea and the lens are two
important structures that help the eyes to focus on objects. The lens is responsible for focusing
the visual image on the back of the eye. The lens changes shape and coordinates with the cornea
to change the direction of the rays of light entering the eyes, hence, focusing the light on the
retina. The retina contains the photoreceptors (rods and cones) that are responsible for
transducing the light into an electrical signal for the brain. The fovea is the part of the retina that
is the most sensitive. In the fovea, photoreceptive cells are abundant. The rest of the retina has
photoreceptive cells as well, but they are not as densely populated as in the fovea, and thus
provide a lower resolution image. The lower resolution region allows for detection of a new
object for targeting, such as when you may first recognize an object in your peripheral vision.
The extraocular muscles rotate the eyeball to focus the point of interest on the fovea. The pupil
opens and closes in response to light intensity. The iris is the colored part of the eye.
There are three pairs of extraocular muscles that control eye movements (Fig 3). The medial and
lateral rectus muscles control movement of the eyes in the horizontal plane. The superior and
inferior rectus muscles control vertical movement of the eyes. The superior and inferior oblique
muscles function to rotate the eye. This torsional movement supplied by the oblique muscles
helps to keep the visual fields in the upright position for small lateral rolls of the head.
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Superior Oblique
Superior Rectus
Lateral Rectus
Medial Rectus
Inferior Oblique
Inferior Rectus
Figure 3: There are three pairs of extraocular muscles that are responsible for moving the eye up, down, left, right,
and torsionally.
These muscle pairs are reciprocally innervated. In other words, they are antagonists. When one
muscle in the pair contracts, the other relaxes. The reciprocal mechanisms allow the eyes to stay
aligned on an object so that the same image appears on both retinas. The extraocular muscles in
the eyes allow for depth perception. Double vision occurs when this mechanism does not
function properly and the two retinas have different focal points. Cranial nerves III, IV, and VI
are responsible for innervating the muscles described above that control eye movements.
The three pairs of muscles that control eye movements are capable of creating many different
types of movements. Fast saccadic eye movements are used to quickly fixate a target. Smooth
pursuit eye movements are used for tracking a target. Vestibular ocular eye movements keep the
eyes focused on a target even while the head is moving. An example of this would be someone
running to catch a ball. Their head moves while they run, yet they are able to stay focused on the
ball. Vergence movements let the eye track near and far targets. The vergence movements are
unique from the other types because they are the only ones that allow the eyes to move in
opposite directions. Optokinetic movements occur when moving through a target filled
environment.
Using the extraocular muscles to generate these kinds of movement, we attempt to place the
target image on the fovea. The eye can then “jump” to focus on the new target, and this jumping
is called a saccade. Saccades can be observed in individuals as they are reading, or looking out
the window while riding in a car. When scanning a visual scene, we make about 3
saccades/second, integrating each of these visual “snapshots” into a high-resolution “mind’s eye”
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view of the scene. Go ahead and watch someone’s eyes as they read this lab. You will see
several saccades per line of text they read.
So what is the exact physical mechanism responsible for generating the EOG signal we measure?
We are not measuring the EMG from the extraocular muscles. Though the exact origin of the
EOG has not been conclusively determined, there are several theories that have been proposed as
the mechanism behind it. The first is the cornea-retinal dipole theory. It states that an electric
dipole is formed through the eye because the cornea is positively charged, while the retina is
negatively charged. As you may remember from physics, a dipole creates an electric field that
can be measured. This is the potential that is being measured by the EOG. As the eye changes
direction, so does the dipole, and thus, the detected signal (Fig 4). The second school of thought
is similar to the one described above, but instead of the dipole being created by the cornea and
the retina, the dipole is believed to be the potential difference across the retina itself. The third
theory states that it is the eyelid movement that creates a sliding potential source, which is
responsible for the potential recording. The cornea-retinal theory is most widely accepted, and
will be used for illustrative purposes in this lab.
Figure 4: A) The cornea is positively charged while the retina is negatively charged. B) This creates a dipole
with an electric field that can be measured.
The EOG is an important signal to be aware of when recording EMG from facial muscles or
EEG. Typical EOG signals have amplitudes in the millivolts range with frequencies of DC 100Hz. We previously learned that EEG signals are on the order of microvolts and that EMG is
in the millivolts range. Since the eyes are located close to the brain and facial muscles, the
different signals may create artifact in one another. EOG can be severely contaminated by the
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EMG signal, and to a lesser degree, by the EEG signal as well. The reciprocal is also true. EOG
can contaminate EEG and EMG recorded around the head and face regions. In some cases, such
as slow eye movements, the EOG can create a DC offset in the EEG signal. These concepts will
be more fully explained in a future laboratory. For the purposes of this laboratory we will be
concerned with measuring the DC component of the EOG signal. The DC component of the
EOG signal can be used to measure eye movement +/- 30 degrees.
As mentioned in the introduction, the EOG is one of the standard biopotentials measured during
a sleep study. This is because a type of sleep of particular interest is called rapid-eye-movement
(REM) sleep. REM involves, as the name implies, very quick and random eye movements. In a
normal night of sleep, REM sleep occurs about every 90 minutes, lasting about 5 to 30 minutes
at a time. This is the time that is usually associated with active dreaming, reduced muscle tone,
reduced cardiac and respiratory rates, and increases in brain activity. Because the increased
brain activity during REM resembles that of a conscious person, REM sleep is also called
paradoxical sleep.
Experimental Methods
Experimental Setup
During this laboratory session you will record three channels of EOG. You will record the DC
coupled component of the horizontal and vertical EOG, plus you will record the AC component
of the vertical EOG. You should view the setup movie included with the CleveLabs software
before beginning the experimental setup for this laboratory session.
1. You will need five snap electrodes for this laboratory. NOTE: For this laboratory, you
may want to attach all of the snap leads to the snap electrodes before you place the
snap electrodes on the subject. It may be uncomfortable for the subject if you apply
pressure to these electrode positions afterwards. Properly prepare and clean the
surface of the skin before applying any snap electrodes (Fig 5).
2. As shown in Fig 5, place one snap electrode below the left eye and one above the left eye.
These electrodes will be used to measure vertical displacement of the eye. Place one
snap electrode to the left of the left eye on the left temple and one to the right of the right
eye on the right temple. These electrodes will be used to measure horizontal
displacement of the eyes. Finally, place the last electrode between the two eyes, just
above the bridge of the nose. This electrode will be used as a ground.
3.
If you have not already, attach snap leads to all of the electrodes. Connect those snap
leads to the harness input channels 1, 2, 3, and the ground using the picture below as a
reference (Fig 5). You will also need to attach jumper wires as shown in Fig. 5. Take all
lead wires and run them behind the ear so the subject has an unobstructed field of vision.
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Figure 5: EOG electrode placements.
Procedure and Data Collection
For each of these experiments, it is very important that the subject keeps their head still while
tracking objects. Only the subject’s eyes should move to track the object.
1. Run the CleveLabs Course software. Log in and select the “Electro-Oculography I”
laboratory session under the Basic Physiology subheading and click on the “Begin Lab”
button.
2. Turn the BioRadio ON.
3. Your BioRadio should be programmed to the “LabEOGI” configuration.
4. Click on the EOG raw data Tab and then on the green “Start” button.
5. You should see data scrolling across the screen. The first two channels are the horizontal
and vertical DC value of the EOG. The third channel is the AC value of the vertical
EOG. You many need to use a very small range (approximately 1uV) on the DC plot to
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see the changes when the eye moves. You may want to use the autoscale axis feature by
right clicking on the scale and turning it on.
6. First we will just examine the signal. Make sure that the time scale is set to about 6
seconds. Then instruct the subject to keep their head still and look all the way up for 2
seconds and then all the way down for 2 seconds. This should give you some idea of the
range of the vertical EOG DC signal. Readjust the range of the plot so you can see it
well.
7. Now have the subject keep their head still and look all the way up for about a second,
then straight ahead for about a second, and then all the way down for a second. Capture a
screen shot of this to your report to show the different voltage levels. Save a few seconds
of data to a file named “verticalEOG” while they are looking up and down.
8. Repeat steps 6 and 7 for the horizontal. Save a few seconds of data to a file named
“horizontalEOG” while they are looking left and right.
9. Instruct the subject to make several blinks and examine what happens to the vertical EOG
AC plot. Capture a screen shot during the blinks. Save a few seconds of this data to a
file called “eyeblinks”.
10. We will now examine how various types of eye movements can impact the EOG signal.
First we will examine fast saccadic movements. You should save this data to a file called
“fastsaccades”. Pick a particular target in the room and instruct the subject to look away
from it. Then, when you tell them to, they should focus their eyes on it as quickly as they
can. You should complete a few trials of this while you are saving data.
11. Next you should begin saving data to a file named “smoothpursuit”. You should save
approximately 30 seconds of data to file while the subject attempts to focus on someone’s
finger tip as they slowly move it around in space.
12. Finally, you should instruct the subject to read a short paragraph in a book or magazine
while you record the eye movements. You should record about 30s of data to file while
they are reading and name the data file “read”.
Data Analysis
1. Using the post-processing toolbox, MATLAB, or LabVIEW, open the saved data file
named “verticalEOG”. Examine the temporal and spectral features of this file on all
channels.
2. Using the post-processing toolbox, MATLAB, or LabVIEW, open the saved data file
named “horizontalEOG”. Examine the temporal and spectral features of this file on all
channels.
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3. Using the post-processing toolbox, MATLAB, or LabVIEW, open the saved data file
named “eyeblinks”. Examine the temporal and spectral features of this file on all
channels.
4. Using the post-processing toolbox, MATLAB, or LabVIEW, open the saved data file
named “fastsaccades”. Examine the temporal and spectral features of this file on all
channels.
5. Using the post-processing toolbox, MATLAB, or LabVIEW, open the saved data file
named “smoothpursuit”. Examine the temporal and spectral features of this file on all
channels.
6. Using the post-processing toolbox, MATLAB, or LabVIEW, open the saved data file
named “read”. Examine the temporal and spectral features of this file on all channels.
Discussion Questions
1. What muscles are responsible for moving the eye quickly in the horizontal direction? In
the vertical direction?
2. Which recording channel is most predominantly affected by blinking and why?
3. Describe the temporal and spectral features that you examined in each of your saved data
files. Explain how these features could be used to automatically detect which type of eye
movement or task is being completed by a subject.
4. How would you calculate the velocity of the eye from the EOG signal?
5. Imagine you are a baseball player standing in the outfield. Suddenly a fly ball is hit to
your right and you begin running after it. You run a short distance, plant yourself where
the ball is headed, wait a few seconds, and then catch the ball. Explain all of the different
eye movements that would have occurred during that sequence of events.
6. Describe several clinical or other applications for which the EOG signal could be
successfully used.
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References
1. Croft, RJ, RJ Barry. Removal of ocular artifact from the EEG: a review. Neurophysiol
Clin 2000: 30: 5-19.
2. Enderle, JD, Blanchard, SM, and Bronzino, JB. Introduction to Biomedical Engineering.
Academic Press, San Diego, 2000.
3. Guyton and Hall. Textbook of Medical Physiology, 9th Edition, Saunders, Philadelphia,
1996.
4. Kandel ER, Schwartz JH, Jessel, TM. Essentials of Neuroscience and Behavior.
Appleton and Lange, Norwalk, Connecticut, 1998.
5. Rhoades, R and Pflanzer, R. Human Physiology. Third Edition. Saunders College
Publishing, Fort Worth 1996.
6. Sadasivan PK, D Narayana Dutt. ANC Schemes for the Enhancement of EEG Signals in
the Presence of EOG Artifacts. Computers and Biomedical Research, 29, 27-40 (1996).
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